refactor(grip): square-only marker set + oriented stretch handles

Phase 1 of bringing the grip vocabulary in line with the standard
CAD convention: drop the per-position-type shape encoding (Diamond
for translate, Circle for parameter) and use a plain square for
every endpoint / vertex / centre grip. The behavioural midpoint vs
vertex distinction stays — it lives on `GripDef::is_midpoint` and
drives `Translate` vs `Absolute` in the grip-edit dispatcher — only
the marker shape collapses.

- `GripShape` keeps `Square`, `Rectangle`, `Triangle`. Diamond /
  Circle are gone.
- `GripDef` gains `dir: Option<[f32; 2]>` — a world-XY direction
  vector consumed by `Rectangle` to orient the box along its
  segment.
- `entities::common` exposes `square_grip`, `center_grip` (same
  square marker, flagged as translate), `rectangle_grip(id, world,
  dir)` for oriented mid-segment handles, and a kept-for-Phase-2
  `triangle_grip`.
- `overlay::GripMarker` / `grips_to_screen[_paper]` thread `dir`
  through to the canvas painter; the Rectangle path rotates the
  box around the grip centre using the world-XY direction (flipping
  the sin for screen-Y).

Every existing `diamond_grip(id, w)` call becomes `center_grip(id,
w)` — the visual change is square-replacing-diamond at line
midpoints / arc midpoints / circle centres / ellipse centres /
viewport / hatch loop centroids / dimension grips / image corners /
multileader / ray / underlay / ole2frame / solid3d.

`LwPolyline` grows a stretch handle on every segment (straight or
arc), drawn as the new oriented rectangle. The grip-edit path
translates both segment endpoints for straight segments and
adjusts the arc bulge from the new midpoint for arc segments.

src/ui/overlay.rs

@@ -26,6 +26,10 @@ pub struct GripMarker {
     pub shape: GripShape,
     /// True → grip is currently being dragged (drawn filled red).
     pub is_hot: bool,
+    /// World-XY direction vector — only consumed by the `Rectangle`
+    /// shape to orient the box along its segment. `None` for grips
+    /// that don't need rotation.
+    pub dir: Option<[f32; 2]>,
 }
 
 // ── Grid display params ───────────────────────────────────────────────────
@@ -424,29 +428,38 @@ impl canvas::Program<Message> for SelectionCanvas {
                     Size::new(h * 2.0, h * 2.0),
                 ),
                 GripShape::Rectangle => {
-                    // Wider-than-tall box — direction-aware mid-segment
-                    // stretch handle.
-                    let w = h * 1.4;
-                    let t = h * 0.7;
-                    canvas::Path::rectangle(
-                        Point::new(sp.x - w, sp.y - t),
-                        Size::new(w * 2.0, t * 2.0),
-                    )
+                    // Mid-segment stretch handle: small box, longer along
+                    // the segment direction so the affordance reads as
+                    // "stretch perpendicular to the segment". `dir` is a
+                    // world-XY direction vector; project it onto the
+                    // screen-X / screen-Y axes implied by the grip's
+                    // 2-D screen position to compute the in-plane angle.
+                    let half_long = h * 1.4;
+                    let half_short = h * 0.7;
+                    let (cos_t, sin_t) = match grip.dir {
+                        Some([dx, dy]) if (dx * dx + dy * dy) > 1e-12 => {
+                            let n = (dx * dx + dy * dy).sqrt();
+                            // Screen Y is inverted vs world Y → flip sin.
+                            (dx / n, -dy / n)
+                        }
+                        _ => (1.0, 0.0),
+                    };
+                    let ax = (cos_t * half_long, sin_t * half_long);
+                    let ay = (-sin_t * half_short, cos_t * half_short);
+                    canvas::Path::new(|b| {
+                        b.move_to(Point::new(sp.x + ax.0 + ay.0, sp.y + ax.1 + ay.1));
+                        b.line_to(Point::new(sp.x + ax.0 - ay.0, sp.y + ax.1 - ay.1));
+                        b.line_to(Point::new(sp.x - ax.0 - ay.0, sp.y - ax.1 - ay.1));
+                        b.line_to(Point::new(sp.x - ax.0 + ay.0, sp.y - ax.1 + ay.1));
+                        b.close();
+                    })
                 }
-                GripShape::Diamond => canvas::Path::new(|b| {
-                    b.move_to(Point::new(sp.x, sp.y - h));
-                    b.line_to(Point::new(sp.x + h, sp.y));
-                    b.line_to(Point::new(sp.x, sp.y + h));
-                    b.line_to(Point::new(sp.x - h, sp.y));
-                    b.close();
-                }),
                 GripShape::Triangle => canvas::Path::new(|b| {
                     b.move_to(Point::new(sp.x, sp.y - h));
                     b.line_to(Point::new(sp.x + h, sp.y + h));
                     b.line_to(Point::new(sp.x - h, sp.y + h));
                     b.close();
                 }),
-                GripShape::Circle => canvas::Path::circle(sp, h),
             };
 
             if grip.is_hot {